This invention relates to well logging methods and apparatus for investigating subsurface formations traversed by a borehole and more particularly to methods and apparatus using electrode systems for determining the average lateral thickness of a zone in the borehole.
When drilling a well, it is customary to use drilling mud containing fine, solid particles in suspension. As the hydrostatic pressure of the mud column is generally higher than the internal pressure of the formations, this mud has a tendency to penetrate into the permeable formations. A mudcake then forms along the wall of the borehole while the filtrate penetrates into the permeable layers, driving out at least partially, the fluid initially contained in these formations. On the other hand, there is practically no mudcake at the level of the impermeable formations into which the mud filtrate does not penetrate. Therefore, knowledge of the presence or absence of this mudcake allows the permeable layers to be differentiated from the impermeable layers.
Moreover, knowing the mudcake thickness makes it possible to determine its influence on the well logging measurements carried out in the borehole. When the mudcake thickness is large, the response of a pad mounted well logging apparatus having a small lateral investigation depth is influenced by the characteristics of this mudcake. A correction may be required in order to obtain the actual characteristics of the formation. In certain cases, the error introduced by the presence of the mudcake may be sufficiently great so as to make the measurements ususable. It is thus very important to determine whether or not the mudcake exceeds a given thickness beyond which a measurement is uncertain and requires correction. In addition, if the mudcake thickness is known with sufficient accuracy, it is possible to correct the measurement and thereby increase the utilization range of a given apparatus.
Various apparatus have been used to obtain information on the mudcake. One of these, described in U.S. Pat. No. 3,462,678 comprises a system of electrodes mounted on a pad. A survey current is emitted into the formation and the resistivities of two formation zones of different lateral thickness are measured. The resistivity of the zone nearest the borehole is influenced in particular by the presence of the mudcake while the resistivity of the second zone which extends deeper into the formation is relatively less sensitive to the presence of the mudcake. Processing circuits generate a signal indicative of the difference or of the ratio of these two resistivities, thereby providing information on the presence or absence of mudcake and an estimation of its thickness.
The measurement obtained using this technique, however, is not very sensitive to the thickness of the mudcake when this thickness is relatively large. Furthermore, these measurements are generally influenced by the contrast between the resistivity of the mudcake, R.sub.MC and the resistivity R.sub.xo, of the zone into which the filtrate has penetrated. It is then necessary to introduce a correction factor dependent on the ratio, R.sub.xo /R.sub.MC which factor can only be approximate owing to the fact that the thicker the mudcake, the more difficult it is to obtain the value, R.sub.xo.
It is also useful to know the average diameter of the borehole. This makes it possible to determine, in particular, the influence of the drilling mud and of the size of the borehole on the well logging measurements. In the past, the diameter of the borehole has been obtained by an apparatus equipped with articulating arms which are applied against the wall of the borehole. The spreading of these arms gives a measurement of the borehole diameter.
Recently, well logging with electrodes of a new type, sometimes referred to as "spherical focussing" apparatus have been made. They are described in U.S. Pat. No. 3,760,260 issued Sept. 18, 1973 to N. Schuster and assigned to the same assignee as this application. In this system, current electrodes associated with A.C. generators emit a main current I.sub.o and an auxiliary current I.sub.1 from a central electrode. One of the current generators is controlled by a control loop connected to two potential electrodes so that the auxiliary current forces the main current to penetrate into the formation. The ratio of the main current I.sub.o to the potential difference E.sub.o, measured between two potential electrodes give a measurement of a conductivity of a formation zone, which measurement is only slightly influenced by the resistivity of the zone (mud or mudcake) located between electrodes and the formation.